The study of genetics has allowed scientists to uncover the origins of life and trace the evolution of different species. DNA, deoxyribonucleic acid, is the genetic material that determines an organism’s characteristics and traits. Over time, evolution has led to the formation of different animal species, including humans.
The evolution of DNA from animal to human has been a topic of interest for scientists for many years. Research has shown that humans share a significant amount of DNA with other vertebrate animals, including mammals, reptiles and fish. According to the National Human Genome Research Institute, humans share similarities of 67% of their DNA with mice, 82% with dogs, and 90% with cats. Additionally, humans share around 98% of their DNA with chimpanzees and bonobos, making them our closest living relatives. Those similarities can help scientists better understand how different species have evolved and adapted to their environments over time and what genetic factors contribute to animal and human diseases. 
Studying DNA evolution from animal to human has allowed scientists to understand better the origins of life and the relationships between different species. Researchers can uncover meaningful insights into the biological processes that underlie various traits and characteristics by analyzing the genetic makeup of various animal classes, like mammals, reptiles, and fish.
In the spirit of World DNA Day, in the following we would like to shine a light on the genetics of mammals, including humans, reptiles and fishes, and give some recent developments around the fascinating world of genetics around animals.
Mammals are a diverse animal class with a wide range of genetic differences. Humans, dogs, cats, elephants, chimpanzees, and blue whales are all mammals, but with unique genetic makeup. Humans share varying degrees of genetic similarity with other animals, depending on how closely related they are on the evolutionary tree. Dogs and cats, for example, are both mammals and thus share a more recent common ancestor with humans than reptiles or fish. Humans have a diploid genome containing 46 chromosomes (23 pairs) in each of their cells, while other mammals can have fewer or more. 
Dogs and humans have lived together for thousands of years, and humans have selectively bred dogs for specific traits such as hunting or companionship. In 2005 the first whole genome of the Canis familiaris, the scientific name for dogs, was published. Dogs typically have 39 pairs of chromosomes, which gives them a total of 78 chromosomes. However, this number can vary slightly between different breeds.
The genome of cats, Felis catus, was fully sequenced in 2007. Most cats have 38 chromosomes or 19 pairs within their cell core. Studies have shown that cats share about 90% of their genetic sequence with humans. While cats have not been domesticated to the same extent as dogs, they have still been living in close proximity to humans for thousands of years and have played essential roles in human society as pest controllers and companions.   
Elephants are more distantly related to humans, as they are part of the order Proboscidea, which also includes mammoths and mastodons. The African elephant, Loxodonta africana, has 56 chromosomes, firstly entirely sequenced in 2009, while the Asian elephant, Elephas maximus, has 52 chromosomes, initially sequenced in 2015. Despite this genetic distance, humans and elephants still share many similarities in their genetic code, particularly in the genes that control brain and nervous system development. It is estimated that elephants and humans share about 70% of their DNA sequences.
Chimpanzees, Pan troglodytes, which are our closest living relatives, share about 98% of their genetic code with us and were initially fully sequenced two years after the completion of the Human Genome Project. They have 24 pairs of chromosomes, making it a total of 48 and just two more than humans. Since their whole genome was sequenced, they have been important models for studying human evolution and disease and have significantly contributed to our understanding of the genetic and physiological factors that distinguish humans from other primates.
Figure 1 – Overview of mammal genetics including the scientific name, number of chromosomes, similarities with humans and when each genomes was initially sequenced.
Genetic testing in mammals
Genetic testing is becoming increasingly important in the care of pet mammals, particularly dogs and cats. By analyzing an animal’s DNA, veterinarians can identify genetic predispositions to certain diseases or disorders, allowing for earlier detection and intervention. For example, certain dog breeds, such as Golden Retrievers, are genetically predisposed to hip dysplasia. By testing for the genetic markers associated with this condition, breeders and veterinarians can reduce the disease’s prevalence within the breed. 
Another important role for genetic testing, particularly in exotic mammals, is improving our understanding of genetic diversity and the conversation of endangered exotic mammals.
Analyzing the DNA of exotic mammals can help conservationists assess the genetic diversity within these populations, which is vital for the long-term survival of a species, as it provides the raw material for natural selection and adaptation to changing environments. By analyzing the genetic makeup of a population, conservationists can identify areas where genetic diversity is low, indicating potential issues with inbreeding or genetic bottlenecks. This information can be used to guide conservation strategies, such as habitat restoration or translocation efforts, to ensure that genetic diversity is maintained or enhanced within populations of exotic mammals. 
Genetic testing can identify individual animals and track their movements, which can be particularly important for exotic mammals that are elusive or difficult to monitor. For example, genetic markers, such as microsatellites or DNA sequencing, can be used to uniquely identify individual animals, allowing researchers to track their movements, estimate population sizes, and monitor changes in population dynamics over time. This information can help guide conservation efforts, such as habitat protection, population monitoring, or anti-poaching efforts, to safeguard exotic mammal populations from threats.
But to get these DNA samples is often very difficult in practice. Most of the time, the areas for these exotic and endangered mammals are deep within the jungle, and thus the search for these animals is like looking for a needle in a haystack. But hope is on the horizon due to very promising results from two recent independent proof-of-concept studies successfully showing, the possibility to collect cell-free DNA out of the air in different zoos and determine, based on specific markers, the correct animals that were present in this area. These filters are at the very beginning of becoming in the future a screening device to look for exotic mammals in wildlife reserves and thus accelerate the search for rare animals and support conservation efforts to save the animal diversity on our planet.
Reptiles are a diverse, cold-blooded animal class, including lizards, snakes, turtles, and crocodiles. Their genomes are significantly different from those of mammals in terms of structure, size, and organization. The difference in genome size between reptiles and mammals is due to the proliferation of repetitive sequences in the reptile genome, such as transposable elements and satellite DNA.
Compared to the human set of 46 chromosomes, the Komodo dragon, Varanus komodoensis, is one example of a reptile that comes closest to it with 20 pairs of chromosomes, making it in total 40. Although the number of chromosomes is close to our set, the Komodo dragons genome only consists of 1.51 billion base pairs which is half of the human one. The Komodo dragon is the largest lizard species in the world, and its genome was sequenced in 2019, providing valuable information about its evolution and biology.
The genome of the American alligator, Alligator mississippiensis, contains approximately 2.16 billion base pairs, wrapped into 16 pairs of chromosomes, which is significantly smaller than the human genome, which contains around 3 billion base pairs. Another interesting fact is that alligators, other crocodilian species, and some turtles and lizards appear not to have sex chromosomes. Instead, the sex of their offspring is determined by the incubation temperature of the egg during embryonic development.  
Tortoises can live the longest of all the land animals on our planet. Estimations indicate that the Aldabra giant tortoises, Aldabrachelys gigantea, shared a last common ancestor about 40 million years ago, while both diverged from the human lineage more than 300 million years ago. Most recently, a research group delivered the worldwide first chromosome-level de novo genome assembly of the Aldabra giant tortoise. This specie’s genome contains 2.37 billion base pairs and is distributed into 26 chromosomes.  
The green sea turtle, Chelonia mydas, has a diploid chromosome number of 56, with a genome size of approximately 2.1 billion base pairs. The genome of C. mydas was initially sequenced and published in 2013 making it the first sea turtle genome to be sequenced. One of the most outstanding genetic discoveries with the green sea turtle genome was the identification of genes involved in the turtle’s extraordinary ability to withstand cold temperatures. Green sea turtles are known to migrate long distances between warm and cold water habitats, which requires them to have a robust thermoregulation system. The genome sequencing revealed that the green sea turtle has a unique set of genes that are involved in the regulation of body temperature and the response to cold stress, which are absent in other reptilian genomes. 
Despite the differences in genome size and the number of chromosomes, studies have shown that reptiles share many of the same genes and regulatory mechanisms as mammals, including humans. For instance, studies have shown that many essential genes involved in embryonic development and limb patterning are conserved between reptiles and mammals. Additionally, reptiles and mammals share many of the same signaling pathways involved in cell growth and differentiation. These findings suggest that despite the differences in genome size and structure, reptiles and mammals share many common features at the genetic and molecular level.
Figure 2 – Overview of reptile genetics including the scientific name, number of chromosomes, size of the genome and when each genomes was initially sequenced.
Genetic testing and genetic methods in reptiles
Genetic testing in reptiles serves various purposes and can provide valuable information, particularly for conservation efforts, species identification, and to shine a light on evolution. However, the databases for reptile genomes are still relatively low. Still, with the decreasing costs of whole-genome sequencing, research efforts worldwide are starting to decode the code of life for one of the earth’s oldest inhabitants, reptiles.
In terms of conservation efforts, genetic testing can help assess the genetic diversity, population structure, and gene flow in reptile populations, which are crucial for conservation efforts. For example, genetic testing can provide insights into the genetic health of endangered reptile species, help identify distinct populations or subspecies that require targeted conservation measures, and guide the management of captive breeding programs to maintain genetic diversity.
Another role of genetic testing in reptiles is precise species identification. Reptiles can be challenging to identify based on morphological characteristics alone. Genetic testing, such as DNA barcoding or molecular phylogenetics, can provide accurate species identification, especially for cryptic or closely-related species. This information is vital for biodiversity surveys, wildlife trade regulation, and forensic investigations involving reptiles. These classifications can then be helpful information to shine further light on how life on land evolved, as some of the reptiles are one of the oldest animals on our planet. 
One of those discoveries was which section of the DNA in reptiles is responsible for their colorization. By using the CRISPR method, functioning as a molecular pair of “molecular scissors” that can be programmed to target and cut specific DNA sequences, allowing for the addition, deletion, or modification of genes, researchers were able to determine the tfec gene as one of the key drivers for colorization in reptiles. The tfec gene is essential for producing iridescent cells in lizards. This discovery highlights the power of gene editing to provide new insights into reptilian biology and advances the knowledge of reptilian genetics. 
Fish, which constitute a diverse group of aquatic organisms with a wide range of forms, habitats, and behaviors, have been extensively studied in genetics to uncover the mysteries of their genetics and evolution. The study of fish genetics has provided insights into topics such as reproductive strategies, population dynamics, ecological interactions, and evolutionary adaptations.
Compared to mammals, fish exhibit significant differences in their genetics. Fish are cold-blooded aquatic creatures with scales and gills for respiration. These distinct characteristics are reflected in their genetics, with mammals and fish exhibiting diverse gene expression patterns, reproductive strategies, and evolutionary adaptations. For instance, mammals typically have highly conserved genomes with relatively low levels of genetic diversity due to their long gestation periods and lower reproductive rates. In contrast, fish often exhibit rapid evolutionary changes in response to environmental pressures, resulting in higher genetic diversity and faster reproductive rates.
In the following, we shine a light on the genetic make-up of different fishes in terms of the number of chromosomes and genome size.
Zebrafish, Danio rerio, are a small tropical fish species that have become a popular model organism in research due to their advantages as they share a significant portion of their genetic make-up with humans, including many disease-associated genes. They are easy to breed, and their embryos develop rapidly and are transparent, allowing researchers to study the genetic mechanisms that govern embryonic development in real-time. Zebrafish have 25 pairs of chromosomes, and their genome is about half the size of most mammalian genomes containing some 1.412 billion base pairs. 
Another fish with historical relevance, as it was the first fish ever to be sequenced, is the Japanese Pufferfish known as well as fugu or with its scientific name Takifugu rubripes. It has a diploid chromosome number of 22, with a genome size of approximately 400 million base pairs. Though the amount of chromosomes is similar to humans, the genome size differs significantly from ours. The result is that the gene density is high, averaging a gene every 6000-7000 base pairs, compared to the human gene density of every 11-15 million bases. The fugu genome was initially sequenced and published in 2002 by a research team led by Dr. Yutaka Suzuki at the National Institute of Genetics in Japan. Sequencing its genome was a significant scientific endeavor due to the unique characteristics of this fish species. Fugu is known for its compact genome, which contains a relatively small number of genes compared to other vertebrates. Yet, it exhibits a high degree of evolutionary conservation with humans, making it a valuable model organism for studying various biological processes. Additionally, fugu is known for its poisonous nature due to the presence of tetrodotoxin, a potent neurotoxin. Therefore, sequencing the fugu genome also provided insights into the genetic basis of toxin production and resistance mechanisms in fish. 
The biggest fish in our oceans is the Rhincodon typus, commonly known as the whale shark. They have a genome size of approximately 3.44 billion base pairs distributed in 102 chromosomes. The genome of Rhincodon typus was initially sequenced and assembled in 2017. The genome sequencing project aimed to provide insights into this iconic species’ genetic make-up and evolutionary adaptations, a species of conservation concern due to its vulnerable status. The genome data has shed light on the evolutionary history, genetic diversity, and population structure of Rhincodon typus and has potential implications for its conservation and management strategies in the face of anthropogenic threats such as overfishing and habitat destruction. The availability of the genome sequence of Rhincodon typus serves as a valuable resource for further research on its biology, physiology, and conservation and contributes to our understanding of the genetic basis of large body size and other unique traits of this majestic species.
Electrophorus electricus, commonly known as the electric eel, has a diploid chromosome number of 52, with a genome size of approximately 700 million base pairs. The genome of E. electricus was initially sequenced and published in 2014 making it one of the first electric fish genomes to be sequenced. The sequencing of the electric eel genome was a significant scientific endeavor due to its unique ability to generate high-voltage electric shocks of up to 600 volts, which are used for navigation, communication, and predation. The electric organ of E. electricus is composed of modified muscle cells called electrocytes, which produce electric fields when activated by the nervous system. The genome sequencing revealed that the electric eel has expanded and diversified gene families involved in ion transport and signal transduction, including the sodium channel gene family, which plays a critical role in the production of electric fields. The sequencing of the electric eel genome has provided insights into the genetic basis of the eel’s unique electric organ and the mechanisms that enable it to generate high-voltage electric fields. This knowledge can be applied to various fields, including bioengineering and biomedicine, where researchers can use the eel’s electric organ as a model for developing new technologies and treatments. 
The sequencing of the electric eel genome has provided insights into the genetic basis of the eel’s unique electric organ and the mechanisms that enable it to generate high-voltage electric fields. This knowledge can be applied to various fields, including bioengineering and biomedicine, where researchers can use the eel’s electric organ as a model for developing new technologies and treatments.
Overall, while there is variation in the degree of similarity between different organisms and the human genome, it is clear that all living organisms on Earth share a common genetic code, with some genetic sequences being more conserved across species than others.
Despite these differences, studying fish genetics is vital to understand their biology and contributing to their conservation. Genetic testing is used to study fish populations, understand their genetic diversity, and inform management decisions.
Figure 3 – Overview of fish genetics including the scientific name, number of chromosomes, size of the genome and when each genomes was initially sequenced.
Genetic testing in fishes or learning more about the inhabitants amongst the world seas
In the case of pets and exotic fishes, genetic testing can be used to identify and diagnose genetic diseases, monitor breeding programs, and identify the genetic make-up of individual fish. For example, genetic testing is used to determine the species of hybrid fish, which can be challenging based on their physical appearance. Furthermore, genetic testing can help monitor the health of captive fish populations, such as in aquariums or fish farms, and to develop better breeding programs to increase genetic diversity and improve the overall health of the population.
Genetic testing has already been used in some fish species, such as rainbow trout and Atlantic salmon, to improve breeding programs and increase disease resistance. In addition, research has been conducted to understand the genetic basis of meaningful traits in fish, such as growth rate and disease resistance. These efforts are important to ensure the sustainability of fish populations and to support the growing demand for seafood in a changing climate.
By Lucas Laner on April 25, 2023.
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